The present invention relates generally to a drive head, such as for a bolt, coupling, nut or the like, particularly those made from metals that are less malleable at forging temperatures and thus require special methods in making to prevent cracking. The present invention further relates to an electrochemical machining method, as well as a tool, useful in making such a drive head.
Fasteners such as bolts or nuts are typically provided with a drive head that can be gripped by the tool, e.g., wrench, so that sufficient torque can be generated to twist or rotate the fastener. The external surface of the drive head can have a number of different configurations. Some typical configurations include square heads (i.e., having four points or corners) and hexagonal heads (i.e., having six points or corners). See, for example, U.S. Pat. No. 3,492,908 (Thurston), issued Feb. 3, 1970, which shows a bolt having a hexagonal head. Other configurations include those having as many as twelve points or corners (sometimes referred to as a “double hexagonal” configuration) to provide the ability to increase the amount of torque that can be imparted by the wrench. See U.S. Pat. No. 3,352,190 (Carlson), issued Nov. 14, 1967; and U.S. Pat. No. 3,354,757 (Grimm et al), issued Nov. 28, 1967. For fasteners such as bolts, the drive head is typically connected to an externally threaded shank. (For fasteners such as nuts, the drive head typically has an internally threaded interior bore and no shank.)
The difficulty in fabricating drive heads for bolts becomes greater as the number of points or corners increases in number. This is particularly the case for bolts where the drive head has a double hexagonal configuration. Relative to a hexagonal configuration, the double hexagonal configuration has a much smaller radius from point to point. This makes it more difficult to provide a double hexagonal configuration in drive head having points or corners that are sufficiently well-defined and sharp so that the wrenching tool can engage the drive head without stripping the points or corners over time.
The difficulty in fabricating drive heads for bolts can also be affected by the ductility (malleability) of the metal, especially at the forging temperature used to make the bolt. Forging temperatures are chosen to be high enough to reduce the energy required for deformation and to reduce the propensity of the metal to crack, but low enough to preclude undesirable metallurgical changes in the metal. For metals having relatively high ductility over a large range of forging temperatures (i.e., are more malleable), the drive head can typically be formed by extrusion, forging or cold forming techniques. These fabrication methods basically form the desired configuration for the drive head by either directly deforming the metal, or by heating the metal either directly (or indirectly due to friction) and then deforming the metal. See, for example, U.S. Pat. No. 3,352,190 (Carlson), issued Nov. 14, 1967. See also U.S. Pat. No. 4,417,464 (Tosa), issued Nov. 29, 1983 (nib tool for cold head forming of a bolt having a hexagonal head); and U.S. Pat. No. 4,023,225 (Tochilkin et al), issued May 17, 1977 (cold shaping of bolt having a hexagonal head). However, for bolts made from metals that need to be forged in a very narrow temperature range, such as powder metal alloys (e.g., nickel alloys containing significant levels of nickel (e.g., at least about 40%) and other metals such as cobalt and chromium), conventional deforming techniques typically used to make bolts have not been found to be suitable. Conventional bolt forging in particular has been found to have a propensity to crack bolts made from less malleable metals, especially when a double hexagonal configuration is formed in the drive head of the bolt.
Other methods that have been used to form drive heads on bolts are electrical (electrode) heating techniques, such as by electrical discharge machining (EDM). See, for example, U.S. Pat. No. 4,473,738 (Wolfe et al), issued Sep. 25, 1984, which discloses an apparatus for forming a polygonal head on the end of a tie rod using a pair of reciprocating electrodes that define a die cavity having walls forming the desired polygonal contour (e.g., hexagonal). Electrical (electrode) heating techniques either etch the surface by moving the electrode so as to melt off material to form the desired drive head configuration, or by generating enough heat from the electrode to melt and deform the drive head within a die having the desired configuration. However, electrical (electrode) heating techniques such as EDM having been found to be unsuitable for forming drive heads from bolts made from less malleable metals, especially those having a double hexagonal configuration. In particular, EDM has been found to undesirably create a large recast layer on the shaped drive head, and can result in reduced material strength and fatigue life for the bolt.
Fabricating drive heads in bolts can be further complicated if it is desired to have an integral flange in the bolt adjacent to the drive head to provide an integral washer or to provide a washer-engaging face. See U.S. Pat. No. 3,492,908 (Thurston), issued Feb. 3, 1970, where bolt 7 has a cylindrical flange 12 adjacent to drive head 11. See also U.S. Pat. No. 3,352,190 (Carlson), issued Nov. 14, 1967, where fastener 10 has a thin integral washer 30 adjacent to drive head 12. This problem of providing such a flange is exacerbated in forming drive heads in bolts made from lower ductility materials, especially if the drive head is to have a double hexagonal configuration where the points or corners need to be well-defined and sharp.
Accordingly, it would be desirable to provide a bolt, nut or other driveable article made from a less malleable metal that has well-defined and sharp points or corners, even when the drive head has a double hexagonal configuration, and having a flange adjacent to the drive head. It would also be desirable to provide a method for making such a bolt that does not have propensity to crack the bolt, to create a recast layer, to reduce material strength or fatigue life, or to impart other undesired properties.